A human civilization, civilization X, decides that food Y is not fit for consumption. Food Y grows well in its climate, is highly nutritious and tasty, and is not harmful to its inhabitants. Food Y could easily become a staple of civilization X, but its people refuse to consume the food. All taboos against food that I have heard of exist because consuming such food would harm the civilization (ex. taboos against horse meat exist in certain cultures because horses are necessary as beasts of burden there). But would a civilization ever create a taboo against a food that would not harm its infrastructure? How and why would such a taboo develop? Are there any real life examples of such a taboo existing and why were they created?

28 Answers
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There are many examples of this in the real world, including the religious bans on pork and shellfish, the historical avoidance of tomatoes, and counter-examples such as the laborious process to transform poisonous raw cassava into something that's not just edible, but a staple in many parts of the world.

In addition, many perfectly palatable foods have a reputation in one culture or another for being only fit for the poor, or even for animals. A classic example is lobster -- sailors' and servants' contracts in the 19th century limited how often they could be given "trash" lobster to eat. The Japanese considered millet and barley only fit for people who could not afford rice. (And not because of gluten intolerance, reportedly rare in Japan.) But that's not the same as a society not exploiting a food source at all.

Some of the reasons for a whole society to develop a complete aversion to a food might be:

(And note: I'm not suggesting everything on this list is correct or rational. I'm just citing real-world examples of plausible aversions. You can tell me raw honey is perfectly safe, but that doesn't change the fact that some people believe otherwise. Tomatoes are an important source of Vitamin C, but were not considered safe to eat in many places.)

For all foods:

It's culturally associated with Those Other People

It's culturally associated with persecution or harassment

People just don't know how. Half of the Donner Party starved to death near a lake full of fish, not yet frozen over when they arrived. The Norse in Greenland didn't fish (disputed); they spent their energy hunting walrus tusks for trade and raising high-status cattle increasingly ill-suited to the land. Chris McCandless starved in the Alaska wilderness because he ate toxic plants and ignored healthy ones (heavily disputed). It takes a lot of trial and error to learn to find the nutritious tubers. This is unlikely to last for more than a few generations, but that may be enough time for the OP's needs.

It takes a lot of work to prepare; it falls out of favor and then people forget how. (Acorns were a staple everywhere oaks grow, but have been mostly forgotten. Leach the flour in several rinses of clean water, or age whole nuts buried in healthy soil.)

For plant foods:

It's easy to make a mistake and accidentally poison yourself:

it's poisonous if not cooked properly (taro, taro leaves)

it's poisonous if not stored properly (ergot infecting rye)

it's poisonous if unripe (elderberries, tomatoes)

it can leach poison out of other things (acidic tomatoes leaching lead out of pewter plates)

it looks too much like something poisonous (many mushrooms, tomatoes)

it often grows wild in places where it bioaccumulates soil toxins, such as heavy metals (or even from the use of tainted fertilizer, as happens in Nigeria.)

it looks creepy (think Halloween eyeball candy)

It's noxious when raw (not poisonous or dangerous, just not something you want around, like durian.)

it's more valuable for something else:

the crop is more valuable, in the long run, as animal fodder or returned to the soil as part of a crop rotation cycle (see below)

the plant is more valuable, in the long run, as food or shelter for beneficial insects or birds

For animal foods:

The animal is too valuable for its labor, fiber, milk, or other resources (cows in India)

The animal is a disease carrier (story 1 for the pork taboo)

The animal is ecologically unsustainable (story 2 for the pork taboo)

The animal is closely associated with Those Other People (story 3 for the pork taboo)

The meat spoils quickly and then causes food poisoning (shellfish taboo from the peoples who brought you the pork taboo)

the animal bioaccumulates toxins (story 2 for the shellfish taboo)

the taboo preserves an important food source for an underclass, indirectly benefiting society as a whole (side effect of pork taboo in Egypt.)

the animal is a companion species

Other ethical reasons, such as a boycott of food produced under inhuman circumstances (veal) or harvested from creature considered too intelligent (whales)

Note that "in the long run" benefits take several generations to establish and thus probably take many generations to disestablish once the benefit diminishes. (Two villages. Village A eats their cover crop in the winter hungry time. Village B has a taboo because their medicine woman once randomly got food poisoning from eating a bad batch//has extreme ecological wisdom, a few vulnerable people starve but the next year's crop is better; there is more food surplus for the next bad year with a hungry time. Village B flourishes, Village A is slightly less successful. After a few generations surplus population from Village B moves to Village A (war if unfriendly, exogamy if friendly) and bring the taboo with them. Now you have 2 villages with the taboo. Repeat until universal.)

A few lists of foods that are poisonous raw, mis-prepared, or when eaten at the wrong time, with examples:

Honey is generally safe but there are a few edge cases, such as babies who haven't developed an immune system yet, or bees that have been feeding on the pollen on toxic plants (and thus a subcase of organisms that can bioaccumulate toxins); see the Cooking stack for more info.

A few examples of cover crops that could provide food at the expense of their primary function:

Brassicas, such as "Forage radish will winterkill and decompose by spring, but it leaves the soil in friable condition and improves rainfall infiltration and storage. It also eases root penetration and development by the following crop."

Cereal grasses (oats, ryegrass, etc.): "Commonly used grass cover crops include the annual cereals (rye, wheat, barley, oats), annual or perennial forage grasses such as ryegrass, [...] A problem common to all the grasses is that if you grow the crop to maturity [...], you reduce the amount of available nitrogen for the next crop. This is because of the high C:N ratio, or low percentage of nitrogen, in grasses near maturity. The problem can be avoided by killing the grass early" (same page as above)

Fava beans: "In the spring, when flowers started appearing on the fava beans, I knew it was time to cut it all down as I didn't want any part of the crop going to seed. If it goes to seed, babies from the cover crop will come up during the summer and compete with the [desired crop]"

$\begingroup$Most of these examples don't count because OP has ruled out cases involving harm (OP even specifically ruled out the example of animals being too valuable for other uses). The only examples that seem to qualify are "it looks creepy", "it looks like something poisonous", and (if we allow animals at all given that OP wants something plant-based) "companion animal". "Ecologically unsustainable" is a possibility but potentially ruled out if OP intended "it grows well in its climate" to also mean it is sustainable.$\endgroup$
– JBentleyJan 27 at 15:46

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$\begingroup$Rye, tomatoes, cassava, etc. are all tasty and nutritious, and all have poisoned people when mishandled. Tomatoes, in particular, were shunned for a long time in many countries. I believe that the historical refusal of the people in many countries to eat tomatoes is quite in line with what the OP is looking for, unless there are clarifications otherwise.$\endgroup$
– arpJan 27 at 17:08

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$\begingroup$I'm skeptical of the link's claims that honey is "filled with poisonous toxins" and needs to be pasteurized or it will sicken/kill you. Pasteurization wasn't developed until the 19th century, but people have been safely eating honey since antiquity. Heck, the Old Testament uses the term "land of milk and honey" to describe an idyllic land full of abundance and eminently suitable for human habitation.$\endgroup$
– Mason WheelerJan 27 at 18:03

$\begingroup$"Closely associated with Those Other People" <== listed under animal-based foods, but this can apply equally well to plant-based, and is IMO one of the best suggestions here. Most other answers are legitimate, practical reasons, but the question seems to be asking specifically for irrational reasons -- why ignore a perfectly good food source? Also note that while "Those Other People" seems to suggest a hated "Other" society, that's not necessary; it could just as well be "Those Filthy Peasants", etc.$\endgroup$
– dgouldJan 27 at 19:49

Something else eats the plant, and that thing is revered.

For example, suppose the people revered a giant bird because it eats rats and thus keeps away plague. If that bird also devours large quantities of the plant during mating season, the plant might become taboo because it is reserved for the bird.

Whatever the reason that the people care about the other creature/species, the food becomes the gift the people provide to that creature/species to keep its favor and to keep it coming near the settlements.

$\begingroup$OP ruled out harm as a reason for avoiding the food. In your example eating the food would be harmful to society because it would increase the risk of a plague. The animal would need to be revered for a different reason (e.g. religion, companion, etc.). Either that, or you'd need to clarify that the food is plentiful enough that humans could eat it without affecting the birds, but choose not to anyway simply because the bird is revered.$\endgroup$
– JBentleyJan 27 at 15:49

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$\begingroup$@JBentley something that promotes health is not the same as something that harms.$\endgroup$
– SRMJan 27 at 17:02

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$\begingroup$I can equally say: "consuming more of the food will cause an increase in disease" (=harm) or "consuming less of the food will cause a decrease in disease" (=promotion of health). Doing the latter is equivalent to avoiding doing the first and is no different than OP's "consuming more horse meat will result in a reduction of beasts of burden" (=harm) which you are arguing is different if the wording is switched to "consuming less horse meat will result in an increase of beasts of burden" (=promotion of productivity).$\endgroup$
– JBentleyJan 27 at 17:43

$\begingroup$@JBentley. I disagree with your claim of equality. Equivalence maybe but not under restrictions I consider relevant to the question. quantamagazine.org/…$\endgroup$
– SRMJan 28 at 1:24

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$\begingroup$@JBentley it's also quite possible that the original source of harm is long gone (society has other means of mitigating rat population) or that the association between the plant and the "revered" animal is wrong in the first place (myth held that the bird ate the plant, but maybe it just liked the leaves for nesting and would inadvertently destroy fruit in gathering them, but is perfectly happy nesting in any sort of broad leaves). Now you have the associative cultural taboo against harvesting the plant without such harvesting causing any quantifiable harm.$\endgroup$
– Doktor JJan 28 at 13:52

Long ago when these groups of people came to settle the land they saw a fruit or whatever. This fruit looks very similar to the fruit of their old homeland that is poisonous. They decide not to eat it. As time goes on people forget the actual reason for not eating this fruit, the fruit become a bad omen . Even more time goes on the local religion thinks the fruit is the sign of the devil or some evil entity. The civilization decides that the fruit itself is evil.

$\begingroup$At one point people assumed tomatoes were poisonous because so many other members of the nightshade family are.$\endgroup$
– arpJan 26 at 8:27

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$\begingroup$@arp well, immature tomatoes are toxic in large quantities since they contain Tomatine. Ripe ones do not.$\endgroup$
– DeepSpaceJan 26 at 14:07

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$\begingroup$Or people eat the wrong part of the plant. Like the poisonous leaves instead of the nutritious tubers of the potato plant.$\endgroup$
– dan04Jan 26 at 15:39

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$\begingroup$This is even more plausible because of mimic evolution. If a plant is toxic to pests, other plants evolve to look like it without ever developing the toxin... just looking like the toxic plant deters pests. Sometimes they’ll also converge on scent. Having two plants that are extremely visually similar in the same or adjacent biomes is somewhat common.$\endgroup$
– SRMJan 26 at 18:20

Because it can't be managed

Out of all the edible plants out there, only some are suitable as large-scale crops. A plant might not be suitable for mass cultivation, or even cultivation at all, not because it's not nutritious and delicious, but because:

it's thorny/brambly and hard, or even impossible, to harvest as a result (raspberries are a niche crop, and rose hips even more so, likely for this reason alone)

it has an unworkably weedy growth habit that causes it to invade neighbors' farm fields (mint, anyone? also, amaranth, dandelions, and quite a few other edible weeds)

it requires soil conditions that are incompatible with other crops (such as highly acidic, alkaline, or saline soils)

its harvest can't be measured reliably for tax purposes (which favors grains and pulses, which are harvested all at once, over roots and tubers, which can be left in the ground to "game" taxes on harvests)

it attracts unwanted guests (such as pests) or serves as a reservoir for diseases (plant pathogens) that attack other crops

$\begingroup$The final reason that you gave, that it serves as a reservoir for diseases, reminds me of the real-life taboo against blackcurrants in the Eastern United States. Older varieties of the fruit spread a disease that harmed pine trees used for logging. Even though newer varieties do not spread the disease and the government allows farmers to grow them, growing the plant is still considered a taboo among older farmers in that region.$\endgroup$
– GalacticJan 26 at 18:36

$\begingroup$I like this one. I'd argue that there is a North American taboo against dandelion.$\endgroup$
– AuspexJan 27 at 17:05

$\begingroup$For a while one of the healthy-foods companies sold an amaranth-based stew that I quite liked, and my local health food store carried amaranth next to the rice and wheat berries, but it's mostly disappeared from the US market except for a few high-priced specialty merchants. Pity that.$\endgroup$
– arpJan 27 at 18:59

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$\begingroup$@fraxinus Yeah -- it went out of fashion when ag was industrialized in the USA, just like how the Industrial Revolution (more or less, don't have an exact date) booted eel off of European plates$\endgroup$
– ShalvenayJan 27 at 23:09

As an example from real life you can look at the Jewish taboo against eating pork. Part of the reason we historically don't eat pork is because pork blood has been used to desecrate the temple (we didn't eat pork before that, but this solidified the animal as absolutely taboo). You could use something similar, have something traumatic happen to your civ that cemented the plant as taboo.

An alternative solution can be taken from what happened when potatoes were first introduced in Europe, unfortunately I can't find the article but I recall reading that one nobleman was accidentally poisoned when his cooks, who never saw potatoes before, cooked the leaves(quite poisonous) instead of the spud itself.

You could use a similar explanation, the first few people to try eating the plant prepared it wrong or ate the wrong part of it and died and so the plant has been deemed inedible

$\begingroup$Wasn't pig blood used to desecrate the temple precisely because it wasn't kosher already then? Which begs the question.$\endgroup$
– leftaroundaboutJan 27 at 13:52

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$\begingroup$@leftaroundabout yes, but this act cemented pigs as extremely taboo, so much so that we can't even farm pigs here for non jews or for export while we can export/farm some other non kosher things, its an order of magnitude different, the ultra orthodox newspapers won't even print the word "pig"$\endgroup$
– NullmanJan 27 at 14:05

The fruit is healthy, but it attracts something that isn't

It could be insects. Disease carrying flies or poisonous bees could be heavily attracted to the fruit. The bees pollinate it, the flies eat it. And when people try to grow and harvest it, the bugs result in deaths.

It could be large predators. Wolves in the area have learned that the smell of the fruit tends to indicate the presence of excellent prey, to the point being drawn to the scent is almost genetic. Cultivating the fruit is like inviting wolves, or whatever dangerous predator is appropriate, to seek out your settlement. The opposite side of the coin is that the fruit might have evolved so its smell attracts the predators, since they prevent herbivores from overeating and destroying the plants.

It could be humans, or other sapients. Even if your locals don't have any reason to revere or avoid the fruit, others might. Another tribe might consider it sacred and attack you. Maybe another group has a slight mutation that makes the fruit poisonous to them. So when your people try to grow it, they think you are cursed, or just trying to kill them and attack. This works especially well if you have other non-Human sapients in your world; i.e. humans can handle chocolate, but to kobolds it is poisonous like it is to dogs.

Perhaps a major figure of an enemy, opposing, or radical ideology used the food in question as a symbol of their beliefs and actions! Similarly to how Hitler used the swastika or christians used the cross! (that's a pair I never thought I'd put together)

Or perhaps the opposite! Perhaps this civilization was the result of a revolution against a group that heavily utilized the food product, leading it to become a symbol of their tyranny. Like the 'prim and proper' clothing of the French monarchs.

$\begingroup$Another good choice. And while there's no taboo against tea-drinking in the US, it's certainly not consumed as widely in the US as it is in Canada—which didn't rebel against its tea-drinking masters. What they have, of course, is largely awful...$\endgroup$
– AuspexJan 27 at 17:12

Maybe some parts of the plant from which the food originates are actually poisonous. If I'm not mistaken this was the case with tomatoes in Europe long ago; the stems and leaves of the tomato plant are poisonous, and so people avoided eating the non-poisonous fruits until, presumably, some brave (or suicidal) soul tried them anyways.

Additionally, it could be that the plant has defense mechanisms other than poison that make people not want to even touch it, let alone eat it. For example, maybe the leaves cause you to break out in hives when you touch them, or maybe they contain a horribly noxious substance that adheres to everything like tree sap. In that case people might avoid growing or cultivating the plant despite it having tastiness and nutritional value.

$\begingroup$“Ur was celebrated by the tribe for his bravery in biting into a pineapple and discovering the golden nectar. Nog was bitter... he’d come so close the previous week by biting into a pinecone.” — source unknown... something I heard long ago.$\endgroup$
– SRMJan 26 at 4:38

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$\begingroup$The reason people though the tomato was poisonous is even more interesting (many common edible foods have toxic parts, like potatoes, so that in itself wouldn't be enough for people to forbid the whole plant). Many plates were made of pewter, which has a high lead content - eating tomatoes, a highly acidic fruit, would cause the lead to leach out of the plates and actually turn them toxic.$\endgroup$
– IndigoFenixJan 26 at 7:51

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$\begingroup$Sorry @IndigoFenix, that sounds like urban mythology. It's highly unlikely that anybody could have connected lead poisoning (a slow and insidious effect) to any specific food item consumed from specific plates. They could, however, have been aware that the tomato is related to Nightshade.$\endgroup$
– AuspexJan 27 at 17:09

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$\begingroup$@Auspex If it is urban mythology, it is certainly widely accepted urban mythology. People could make connections, right or wrong, between people who enjoy tomatoes and the highly visible effects of lead poisoning over time, especially since tomatoes tend to be a love-them-or-hate-them kind of food. Though the fact that it is related to nightshades no doubt had a part to do with it as well.$\endgroup$
– IndigoFenixJan 27 at 20:00

It has an unfortunate appearance, which results in it being associated with other taboos.

For example, if your people have a taboo against eating the Sacred Cows, then this particular fruit looks like a cow's head. If your people have a nudity taboo, it resembles body parts that are traditionally covered up (and, in most of those cases, even more so if it contains or exudes a milky white liquid)

Some of these taboos might make the food completely forbidden (consider the violence in parts of India against people merely accused of eating beef). Others might just relegate it to shunned subsets/castes of society ("only scarlet women eat the 'male fruit'")

There are two mayor causes this taboo could have been originated from.

While not harmful, the food does something with your body with a negative connotation.

The Food is in direct or indirect connection to a source of harm

To the first point you said this:

A human civilization, civilization X, decides that food Y is not fit
for consumption. Food Y is plant-based, grows well in its climate, is
highly nutritious and tasty, and is not harmful to its inhabitants.

While not harmful to the population, it could cause some changes, that are unwelcome. For example the food could have a similar effect as asparagus or garlic, making it unfit in society to eat the food. Of course this effect would need to be way stronger than that of my two examples provide, but it could be possible.
Depending on the setting, in a more fantasy style it could change the eye color of a person to a devilish red. While harmless, the inquisition would certainly prevent people of eating it in fear of hellish infestation.

The second is a more indirect cause. While the food is perfectly tasty and healthy, this would also mean it would be that for other animals aswell. The food could be the main food source for species of rodents or bugs, maybe even using the food as a breeding ground (like apples for worms).
While farming and trading the food, the pests travel with it. Imagine the first delivery of those foods and a wave of rodents enter the city with it. Or poisonous bugs and spiders attacking their 'living room'.
While in modern society, this would be treated with delicate methods, back in the days, this could cause superstitious farmers and villagers to never touch them again.
On a related note, this is why eating bugs is considered disgusting in the western world, as they crawl through dirt and wastes in the wild, transmiting diseases. In truth, the bugs for eating are raised pretty sterile and certainly without dirt and waste.

The food is a main export of a nation or company and there is heavy disinformation against it, or many patents / gmos invoked with it and some economy is heavily dependent on it (like soy), leading to economic war and association of the food with that enemy nation or culture.

$\begingroup$I really like this answer; it is related to a kind of shrubbery in my mythical world (fantasy mostly) that grows a berry that is directly tied to the weaving of goat/sheep wool into high quality fabrics, and the goats/sheep eat it. I will now add a taboo for certain sub species of that plant, since a few of the berries, the blue and the red, are fermented to make liquor.$\endgroup$
– KorvinStarmastJan 26 at 15:36

$\begingroup$@KorvinStarmast Making liquor is indeed holy work. If the people in your story are dwarf/norse type people, the priests of the liquor god will defiantly smite whoever speaks of eating the sacred berries!$\endgroup$
– Nahshon pazJan 27 at 8:39

$\begingroup$And smite them in the name of the rye, the corn, and the whiskey barrel .... X^D$\endgroup$
– KorvinStarmastJan 27 at 13:31

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$\begingroup$This answer doesn't make sense. If it's cheap enough to be used for electricity production, then this will certainly not be a hindrance for use as food. (At worst it will make it a low-prestige food, similarly perhaps to how many Americans consider soy unworthy animal fodder.)$\endgroup$
– leftaroundaboutJan 27 at 14:00

$\begingroup$If part of a food get grown for critical industries there's strong economic pressure to use it for other purposes as well. Economic pressure are often at the beginning of a taboo and later it gets justified by religion.$\endgroup$
– ChristianJan 27 at 14:53

Hi welcome on Stack Exchange. I have a suggestion for what you are searching for: Adam's infamous "apple."

Imagine that in your civilization, priests are quite important and are able to impose taboos in the culture of the civilization. Then, imagine that the myth of the foundation of humanity in your civilisation is the Adam & Eve narrative, with the "apple" replaced by food Y.

Then why should your people eat something that is the symbol of a bad thing, a.k.a. how the humanity had been sentenced by its God(s)?

$\begingroup$The fruit was not necessarily an apple, and there is no taboo on apples in Christian practice. (But I like your answer for the reasoning used to establish a taboo).$\endgroup$
– KorvinStarmastJan 26 at 15:41

$\begingroup$@KorvinStarmast You're perfectly right (though I did not know the fruit was not necessarily an apple), but a similar myth could create a taboo in an established and more strict religion$\endgroup$
– totalMongotJan 26 at 16:29

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$\begingroup$@KorvinStarmast I have actually attended church in two distinct and unrelated communities in which the Christian ministers did in fact ban apple consumption due to said tale. It's certainly not a common practice, which I have not heard of outside of those two communities. But your comment reads like it never happens, and I can say with conviction that it does, just not often or many places.$\endgroup$
– Ed GrimmJan 26 at 20:18

The ruler needs to control his people. To control his people, the ruler must control the food supply.

The Umatupu plant, being highly nutritious and growing well, severely impacts the ruler's ability to control the food supply. Lucky for him, the clergy have deemed the Umatupu plant as taboo because it is the mysterious plant mentioned in section four of the Holy Book. The clergy finds favour with this ruler.

Water is plentiful. It falls from the sky, is piped into most homes at a rate that is nearly free for drinkable quantities, and is quite safe to drink in most developed areas. Yet there are many people who won't touch tap water unless you pour it in a plastic bottle with a brand on it and sell it to them for a few bucks.

One reason could be because it resembles other things that are dangerous to us.

Tarantulas are mostly pretty harmless but we don't eat them possibly because they resemble other highly venomous species like the brown recluse. Eight legs, lots of eyes, etc.

Kind of related to what causes batesian mimicry. Milk snakes (non venomous) evolved to look a lot like coral snakes (highly venomous) so they are less likely to get eaten because predators that like eating brightly colored snakes tend to die a percentage of the time.

Not exactly the same, but ancient Hawaiians had taboos (Kapu is actually where the word taboo comes from!) revolving around certain foods. Women were not allowed to eat pork, bananas and coconut as they related to parts of the body of the gods, nor were they allowed to eat with men. These beliefs fell (violently) out of favor after King Kamehameha II shared a meal with his mother and others and not being struck dead.

If this civilization has a deeply rooted religious or otherwise superstitious belief in the origin, or some other meaning behind the food, then perhaps they would not eat it due to continuing societal peer pressure.

$\begingroup$"Taboo" actually comes from "tabu" - a word from Tonga (about halfway between Hawaii and New Zealand). However, the word was introduced into English by Captain Cook, who also 'discovered' Hawaii for Europeans$\endgroup$
– ChronocidalJan 27 at 15:28

$\begingroup$Ancient Hawaiian was very similar to those other Polynesian languages. When Hawaiian was romanized the "T" sound was eventually transitioned to a "K" sound, so pre-discovery, the word would have been tabu, but is today pronounced Kapu. It is kind of interesting to me the idea of having ones language changed by an outside party like that. Unfortunate, but interesting.$\endgroup$
– CodeWarriorJan 28 at 22:02

If the seed pods looked a little bit like little babies, then it could be very understandable that there would be a taboo against eating it. In fact, natural selection could well produce something like that if the seed pods that looked the most like babies was left alone.

There's a Japanese crab that has the face much like a Samurai that may have evolved in this manner.

The food is widely known as feed for farm animals. So eating it would be equivalent to someone today who eats cat food.

A culturally influential region might have seen very high demand for meat for a long time. It was more profitable to feed the food to a pig and sell the pork than to sell the food directly. Because of this people saw it on sale less and less until it was only known as animal feed.

Desperately poor people would still resort to eating or stealing it, so it becomes associated with crime, poor morality etc. and then Kings and religious leaders condemn and suppress it.

Slavery is a widespread practice. The slaves are kept servile by use of a mind-numbing drug. This plant counteracts the effects of the drug, thus threatening the social order.

Overpopulation threatens the world. To survive in the absense of this plant requires a large, expensive basket of foods, some of which must be imported from distant lands, and that only the elite can afford. Again, cultivating this plant threatens the social order.

I came up with two ideas that would not need too much suspension of disbelief from a casual reader.

1.) The fruit is worshiped religiously

Saint X of place Y was once starving, when he discovered "the fruit". He ate the fruit, and he was saved. But we are not Saint X of place Y, and we are not worthy of eating "the fruit". Only the saints may, and you and I are no saint.

2.) The fruit makes you feel good

Long, long ago, in the old kingdom, people ate "the fruit" regularly. It gave them a feeling of comfort and warmth, any they loved it. "The fruit" was added to every dish, and juice from "the fruit" was bottled up and sold. As time went on, the lives of people began to revolve solely around "the fruit", until they ceased all productive work and only ate "the fruit".

The society collapsed inadvertently. Those who had access to "the fruit" tried to take control over the masses, and the masses revolted. It was bloody, and only few survived. Those who did made a pact never to eat "the fruit" again, and the civilization that followed would heed their warning never to eat "the fruit".

The plant releases a toxin that pollutes the soil, and nothing can grow there for 10 years(or etc)

The plant absorbs all the nutrients from the soil, and nothing can grow there.

Requires too much water, or etc.

It has too many nutrients say 40mg of iron, but an adult can only safely handle 45mg/day. Now having 1 serving limits how much other food you can have. Maybe it has 70mg, now you have a real problem.

Combine last one with serving size, maybe you can only have 1 oz a day, and it fills ALL your daily requirements. However, the serving size is 1oz so you still feel hungry, but if you eat 1 more thing you overdose on something. Want a piece of cake,candy, pizza, or etc, nope if you do its off to the hospital.

Because it also gets you high. While not physically harmful or addictive in itself, the food is shown to produce artificially elevated mood, lowered inhibitions, decreased fine and gross motor control, and mild hallucinations. These mind-altering effects have been responsible for notable injuries or deaths of those who eat the food or those around people who do, and as a result the culture has criminalized the consumption of this foodstuff. Marijuana is nominally illegal in the United States and many other countries for this reason, even though most consumption of it is less physically damaging to the body than alcohol, and the seeds have been found in USDA studies to be a better source of protein than soybeans.

Because the plant or animal involved has religious significance. It signifies some blessing of the land and people by a deity of the dominant religion of the civilization, and it is considered a terrible sacrilege to defile the plant by harvesting it, or the animal by killing it for meat. Alternately, the animal or plant is considered "unclean", and again it is sacreligious to eat one. Cattle in Hindi cultures is the foremost example of the first option, while pork in Jewish and Muslim cultures is an example of the second option.

You can combine this with the first option, to make a plant or animal sacred and its consumption reserved for religious observance, due specifically to the physical effects produced by consuming it. Marijuana is consumed religiously in Hindi cultures, and highly illegal outside this use. Alcohol may be given to minors in the US only in accordance with religious observances, such as Communion/Passover wine (in modern usage it has little intoxicating effect). Juvenile buttons of the peyote cactus are used religiously and medicinally by pre-Columbian civilizations in the present-day American Southwest and Northern Mexico.

Because it is easily confused with a related species that is highly toxic. Consider a close cousin of a tomato-like fruit, the plant and fruit both all but indistinguishable to the casual observer when the fruit is ripe (you may be able to tell the difference by inspecting immature fruit). One variety is harmless, nutritious and delicious, while the other is nicknamed "last pucker", for an extremely sour, bitter taste, that tells the unlucky eater they are already dead. Even spitting the bitten part out and washing out the mouth isn't enough; the toxin is readily absorbed through the skin of the tongue, and even small doses are a fast-acting nerve block that targets autonomic nerves of the cardiopulmonary system and brain stem. The toxin is harmless to birds (the target species for seed dispersion), but deadly to mammals. As such, the civilization eschews both species as possible direct food sources (though the toxin makes an extremely effective weapon), as the risk is just too great given plentiful sources of other, much safer foods. Real-world examples, most not quite so drastically toxic or so indistinguishable from a safe species, include many mushrooms, certain berries, and a few poisonous animals including fish like the fugu puffer fish, and amphibians like the cane toad and the poison dart frog.